WASHINGTON — The Obama administration expressed growing concern Friday over Russian intentions in Ukraine, with Secretary of State John Kerry delivering a blunt warning to Moscow against military moves in the country’s southern Crimea region that could further inflame tensions.

As pro-Russia gunmen patrolled Crimean streets in armored vehicles and took over airports there, Kerry said he called Russian Foreign Minister Sergey Lavrov for the second time in two days to press the Kremlin to hold to its promise to respect the former Soviet republic’s sovereignty and territorial integrity.

Kerry told reporters that Lavrov had once again repeated Russian President Vladimir Putin’s pledge to do just that, while also pointing out that Russia has broad interests in the former Soviet republic, including a major naval base in Crimea. But Kerry, in comments that highlighted Washington’s rising suspicion of Moscow, said the U.S. is watching to see if Russian activity in Crimea “might be crossing a line in any way.” He added that the administration would be “very careful” in making judgments about that.

“While we were told that they are not engaging in any violation of the sovereignty and do not intend to, I nevertheless made it clear that could be misinterpreted at this moment,” Kerry said. “There are enough tensions that it is important for everybody to be extremely careful not to inflame the situation and not send the wrong messages.”

Ukraine, meanwhile, accused Russia of a “military invasion and occupation,” saying Russian troops have taken up positions around a coast guard base and two airports in Crimea.

Kerry reiterated the U.S. view that Russian military intervention in Ukraine following the ouster of the country’s Russia-backed leader would be “a very grave mistake” and could run counter to Russia’s self-professed opposition to such operations in other countries, such as Libya and Syria.

And, Kerry noted that during his call with Lavrov, fugitive Ukrainian President Viktor Yanukovych was holding a news conference in southern Russia in which he said he was not asking Moscow for military assistance and called military action “unacceptable.” In his appearance before journalists, however, Yanukovych also vowed to “keep fighting for the future of Ukraine” and blamed the U.S. and the West for encouraging the rebellion that forced him to flee last weekend.

Any Russian military incursion in Crimea would dramatically raise the stakes in Ukraine, which is at the center of what many see as a Cold War-style tug-of-war between East and West. One of the catalysts for massive demonstrations that led to Yanukovych’s ouster was his rejection of a partnership agreement with the European Union in favor of historical ties with Moscow. That EU agreement would have paved the way for Ukraine’s greater integration with the West, including potential affiliation with NATO, something to which Russia strongly objects for former Warsaw Pact members.

Kerry and other senior U.S. officials have tried without success to dispel widespread sentiment in Russia that the United States and Europe are trying to pry Ukraine out from under Russian influence. On Friday, Kerry said issues like EU or NATO partnerships should be put on the back burner in order to concentrate on reducing tensions and setting up a democratic transition.

“”We do not want to get caught up in the historical or the more current tensions over association agreements or NATO or other kinds of things,” he said. “There’s a place for that down the road if Ukrainians want to have that debate, but we do not believe that that should be part of what is happening now. Now is time for transition and for respect for the pluralism and diversity and democracy that the people Ukraine want.”